140 Species and Species Change 



in rate of change hold true between phylogenetic Hnes m existence 

 now. A satisfactory theory of evolutionary change must, therefore, 

 provide for both stability and change. Such a dual explanation 

 must take two circumstances into account : ( 1 ) that the amount of 

 variation in a species is usually low and (2) that any phylogenetic 

 line maintaining viable populations today is well adapted to the 

 ecological situation within its range. 



STABILITY IN ADAPTATION 



Considering the dynamic nature of both phylogenetic lines and 

 the environment, it might seem as if change should be the rule 

 and stability the exception. In many groups of organisms the op- 

 posite is found. 



In the caddisfly tribe Philopotamini the primitive and near- 

 primitive forms include 90 species, all occurring only in cold water 

 streams; from this complex set of lineages (Fig. 58), dating back 

 probably to the Cretaceous, only two surviving lines became adapted 

 to warm water streams (Ross, 1956). Even greater stability is found 

 in the tropical caddisfly genus Leptonema in which some 80 known 

 species have evolved during the Cenozoic, but none has yet be- 

 come established more than a few miles beyond tropical areas. 

 If the phylogenetic lines of the genus Leptonema could be joined 

 end to end, the result would represent something like three billion 

 years of continuous genetic reproduction without more than minor 

 and local adaptive changes. Comparable stability is suggested by 

 the great age of many living marine genera, many tracing back 

 over 400 million years with little structural change and probably 

 little ecological change (Schrock and Twenhoffel, 1953). These 

 examples give credence to the idea that, in evolving phylogenetic 

 lines, stability is the rule and that some unusual circumstance is 

 necessary for the line to change. 



Stable phylogenetic lines appear to remain within the same 

 ecological range, in other words, to live under nearly identical 

 conditions generation after generation. In the coelenterates, Bayer 

 (1955) reported from the lower or middle Eocene of Trinidad 

 remarkably well-preserved fossil sea pens Virgularia presbytes 

 which are apparently the same species as recent populations in the 

 Gulf of Mexico. This phylogenetic line thus seems to have per- 

 sisted without change in the same general area and presumably 

 under the same ecological conditions for some 40 or 60 million 

 years. Even so, most if not all ranges encompass some average 

 ecological variation and have local and periodic ecological oscilla- 



